Ceramic article and method of making same



Patented Feb. 10, 1942 Tainc G. McDougal, Karl Schwartzwalder, and

Albra H. Fessler, Flint, Mich., assignors to'General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Application May 9, 1940, Serial No. 334,266

3 Claims.

This invention relates to ceramic bodies having compositions which lie in the ceramic oxide system BeO-AlzOs-SiOz and to methods of making articles of such compositions. Such compositions are particularly adapted for use as insulators for spark plugs and the like which require one or more of the physical characteristics of high thermal conductivity, high thermal capacity, high thermal expansion, high dielectric strength at elevated temperatures and the ability to withstand sudden temperature changes. All of these physical characteristics are prerequisites of satisfactory spark plug insulators for automotive and aircraft engines.

Spark plug insulator bodies made according to this invention possess the indicated qualities to a high degree and in addition possess the great advantage that they may be fired at relatively low temperatures. This is important in commercial production as the equipment required to fire at high temperatures is both difficult and expensive to construct and maintain.

We have discovered that certain combinations of oxides in the triaxial system BeO-AlzOs-SiOz produce very excellent insulators at temperatures only slightly higher than those required for firing present-day commercial porcelains.

The preferred bodies have compositions falling within the following range:

These bodies are marked by decreased sintering temperatures, high thermal conductivity, high thermal capacity and high thermal expansion without thermal shock breakage.

The following examples indicate the relatively low sintering temperatures obtaining:

A body having the composition 40% beryllium oxide, 50% aluminum oxide and silica was successfully fired at Orton cone 2'7 (1650 C.)

A body having the composition 30% beryllium oxide, 50% aluminum oxide and 20% silica was successfully fired at Orton cone 20 (1530 0.).

Spark plug insulators made from these new compositions have such greater thermal conductivity and thermal capacity that, in measurements in terms of insulation length required for spark plug insulators, they are six to seven insulation lengths better than the best grade of spark plug porcelain. That is, an insulator of the new composition 6 or finds of an inch longer than the best porcelain insulator will perform as well as such porcelain insulator with respect to the said thermal qualities. They are also superior in this respect to any of the increasingly used sintered oxide insulators with which we are familiar. At the same time no breakage occurs due to thermal shock and the electrical resistance at high temperatures is considerably higher than that of porcelains now in general use. The thermal expansion is also greater, an advantage in that it more nearly approximates that of the metal housing in which the insulator is assembled to form the complete compression-tight spark plug. It is indicated that these new compositions may provide as great improvement for such uses as radio work where low power factor loss is necessary as they do for spark plug insulators.

The materials required for the bodies may be added as oxides or as various compounds or compositions such as aluminates, and in different mineralogical forms. It will be understood that where in the claims the bodies are defined by oxide content such variations are contemplated as equivalents. It will also be understood that small proportions of suitable known ceramic fluxes or diluent materials may be added when desired in accordance with well-known practices in the ceramic art. The following are some examples of the forms in which the raw materials may be introduced into the batch:

Beryllz'a Beryllia, preferably low in alkalies, may be added as the chemically precipitated beryllium oxide, raw or calcined; or as synthetic beryllium alurninate.

Alumina Alumina is preferably added as calcined alumina prepared by the Bauer process; as calcined diaspore or bauxite; as electric furnace or high temperature calcined alumina.

Articles produced from the compositions specified may be processed in the manner described in U. S. Patent 2,091,973, granted to Fessler and Russell on September '7, 1937, or in our Patent No. 2,122,960 granted to Schwartzwalder on July 5, 1938; or by casting in molds; or by extrusion and grinding the dried extruded blank. The usual organic plasticizers or inorganic gels or zeolites may be added to aid fabrication.

The raw materials employed in the body compositions should be in a very fine state of subdivision, preferably all the material finer than 43 microns with a greater portion lying between 0 and 5 microns.

In our preferred method of forming, disclosed in said Schwartzwalder patent. the inorganic materials are ground with a suitable proportion of temporary organic binder. such as Bakelite, together with a lubricant. The finely ground material is then granulated and preformed into predetermined shapes. These shapes are then assembled on a center pin placed in the die and pressed into insulator shapes under heat and heavy pressure. Setting of the binder produces a firm shape readily handled in mass production facilitating the subsequent manufacturing operations. The formed body is fired to a aumciently high temperature to eliminate the organic binder and to recrystallize the mass into a fine-grained dense impervious structure.

We claim:

1. A ceramic product made by firing a ceramic mixture consisting of from .35 to 99% beryllium oxide, from .50 to 96% aluminum oxide and from .50 to 30% silicon dioxide.

2. An insulator for spark plugs and the like characterized by high thermal conductivity, high thermal capacity, high thermal expansion, high dielectric strength at elevated temperatures, high thermal shock resistance and good mechanical strength made by firing to a dense, non-porous state an insulator shape formed from a batch TAINE G. MCDOUGAL. KARL BCHWARTZWALDER. ALBRA H. FESSLER. 

